The present invention relates generally to nuclear reactor safety systems and, more specifically, to such systems that operate in a passive manner to remove decay heat.
Cooling systems are of vital importance in nuclear reactors. In a pressurized water reactor (PWR), coolant water circulates through the reactor vessel containing the nuclear fuel and then through the primary side of a heat exchanger, which produces steam in a secondary loop for powering turbines. In a boiling water reactor (BWR), water in the reactor vessel is converted into steam that is used directly to drive a turbine.
In the event of an accident, the reactor may need to be shut down. To initiate shutdown, control rods are inserted into the core. However, the reactor continues to produce decay heat after the control rods have been inserted. The removal of decay heat is the essential safety function in a nuclear reactor after shutdown. If the cooling systems are damaged, the reactor may accumulate excess heat that could fail the fuel elements in the reactor vessel, leading to a release of radioactive materials. Passive cooling systems reduce the probability of fuel damage because of their high reliability and independence from electric power.
Heat removal systems, commonly known as "safety condensers" or "isolation condensers," can remove the decay heat in a passive manner. Such systems use natural convection to condense the steam in a heat exchanger. This form of convection is the same as that which creates ocean currents, weather patterns, and flames, and is commonly known as hydrostatic natural convection or aerostatic natural convection. In general, static natural convection is caused by pressure differences created by heating a fluid under the influence of gravity.
One disadvantage of the use of static natural convection for passive heat removal in nuclear reactors is that the condenser must be located at an elevation above the steam source. Thus, such systems impose serious physical design constraints that add to the cost of the reactor. Furthermore, any non-condensible gases that enter the system can accumulate in the condenser, reducing heat transfer and eventually blocking hydrostatic convection. It is known that hydrogen may enter the system when heat released during an accident causes water to react with metal, and nitrogen may enter the system when emergency coolant from nitrogen-pressurized tanks is injected into the reactor vessel in the event of a malfunction of the primary cooling system. Steam traps with float-actuated valves cannot be used to remove non-condensible gases from the current condensers because they cannot separate the gases from the steam under the transient conditions of an accident.
In the event of damage to the primary cooling loops, emergency systems may be used to add coolant, as disclosed in U.S. Pat. Nos. 4,444,246 and 4,280,796, both issued to the inventor of the present invention. The system described in the above-referenced patents is passive, i.e., it operates without mechanical pumps and emergency power supplies. When coolant water is initially added to a hot reactor vessel, the resulting eruption of steam can cause steam binding, which prevents the flow of coolant water into the vessel. The above-referenced patents describe a system for alleviating the steam binding problem by using a condenser loop to quickly remove the excess steam. The system is not designed for long-term decay heat removal. In that system, a jet pump condenses the excess steam and returns the condensate to the vessel. Such a jet pump is a well-known device that injects steam through a nozzle at sonic or supersonic velocity into water in a mixing tube. The steam condenses rapidly, and the resulting compression shock creates suction to draw additional water into the mixing area.
Because jet pumps for steam binding alleviation applications involve rapid responses to an emergency, such as adding coolant water and condensing steam, these jet pumps are designed to condense the maximum amount of steam in the shortest period of time, Thus, these jet pump designs have design parameters including steam nozzle areas that are large relative to the water intake area to provide a large amount of steam relative to water. For long-term decay heat removal, however, the same design parameters would inhibit efficient removal of heat from a loop containing a heat exchanger through which the jet pump circulates water because the loop temperature fluctuates substantially with transient changes in steam pressure and temperature during the accident. Heat removal efficiency is maximized when the temperature of the loop remains high relative to that of the heat sink in which the heat exchanger is placed. Thus, if a long-term decay heat removal system were to use such a jet pump, after prolonged heat removal the heat sink temperature would 'exceed levels at which heat removal is efficient, and the steam would cease to completely condense.
A high steam pressure head is required to power a jet pump such as that described in the above-referenced 'patents, and such pressures exist only for a short duration after emergency cooling water has been dumped into the reactor vessel. The steam-binding alleviation system described in the above-referenced patents is not designed for long-term heat removal because it is only required during refilling of the reactor vessel; refilling takes only minutes, but decay heat must be removed for hours or even days.
An economical passive heat removal system that can continue to operate efficiently throughout the range of temperatures and pressures generated in a reactor during an accident or after shutdown would be highly desirable. Such a system should be economical and retrofittable to existing reactor designs. It would also be desirable for such a system to remove non-condensible gases. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.